Add support for custom headers in AsyncWebsocket

[p-i-]

I've updated Gin AsyncWebsocket to support custom headers when creating a websocket connection, so I can now replicate what I was doing in Python, i.e.:

ws = create_connection(
    WS_URL,
    header=[
        f'Authorization: Bearer {API_KEY}',
        'OpenAI-Beta: realtime=v1'
    ]
)

In C++:

    juce::String apiKey = "...";
    
    juce::URL openAIURL("wss://api.openai.com/v1/realtime?model=gpt-4o-realtime-preview-2024-10-01");

    juce::StringPairArray customHeaders;
    customHeaders.set("Authorization", "Bearer " + apiKey);
    customHeaders.set("OpenAI-Beta", "realtime=v1");

    websocket = std::make_unique<gin::AsyncWebsocket>(openAIURL, customHeaders);